Correlation of regenerated fibres morphology and surface ... - Lenzing

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Lenzinger Berichte, 82 (2003) 83-95 surface tension, sorption velocity, contact angle and finally the polar and disperse part of surface free energy. Structure of man made cellulose fibres Man-made cellulose fibres are produced from regenerated cellulose II which is obtained in two ways, either treating native cellulose with strongly alkaline solutions or precipitation from solutions. These fibres are generally described by a fibrillar structure model [15]. Depending on the spinning conditions, differences in the structural arrangement between different types of man-made cellulose fibres [9,20,9,28] are found, for example different density, crystallite size, crystallite orientation and pore structure. Fibres spun according to the viscose process (CV) have a skin-core structure and a transverse non-uniformity. More uniform structured and spherical crosssectioned modal fibres are obtained [9] if a modified viscose process retarding the cellulose regeneration is applied. This process causes increased molecular orientation especially in the crystalline regions and higher stretching ratios compared to the conventional viscose process, leading to high tenacity fibres. NMMO or lyocell fibres (CLY), produced by the amine oxide process [9] are solvent spun and show characteristic round cross-sections and a smooth surface. Their high degree of crystallinity, molecular orientation and higher molecular weight causes special properties such as high wet fibre strength and comparatively low elongation, leading to the remarkable dimensional stability of fabrics produced from them [32,9,18,23]. No significant differences in the crystallite dimensions are reported, nevertheless the degree of crystallinity differs [9]. Lyocell fibre’s degree of crystallinity is 16 % higher than that of modal fibres and much higher (43%) compared to viscose fibres. The fibrils in lyocell fibres are thinner than in CMD and CV fibres and molecular orientation is nearly the same (orientation function of lyocell fibres is 3% higher) as in modal fibres but much higher 84 compared to viscose standard fibres (18%). Lenz and Schurz [15] determined the size of crystalline regions of regenerated cellulose fibres: length 12 - 14 nm, width 8 - 10 nm, and thickness 3 - 4 nm. The broad plane of the platelet-shaped crystallites is situated parallel to the 101- lattice plane. The crystallites form strands with a length of 150 nm - 550 nm. They are partly bundled up in clusters with diameters of 30 - 60 nm, partly separated by less dense spacing regions. Surface Tension The above discussed crystallites size, orientation and distribution as well as the presence and size of amorphous regions and voids (void volume, inner surface) [19] are the parameters that determine the fibres adsorption and interaction abilities. Polar and non-polar groups cause fibre’s hydrophobic and hydrophilic nature obtained by various treatments. A lack of hydrophilicity is either due to the absence of polar groups or due to their non-accessible locations. The degree of hydrophilicity or wetting of a nonporous solid by a liquid is observed in two ways, either measuring the contact angle θ directly using goniometry or calculating it from data obtained by tensiometry. From the known contact angle or the help of indirect methods [3,39] the surface energy of solid can be determined [3,25,27,36,40] and separated into a polar and a disperse part, according to the following considerations: If a liquid and a solid surface come in contact, an interaction between the polar parts as well as the disperse parts of both phases will take place at the interphase, but not between the polar and the disperse parts [14]. The interfacial tension of the two phases in contact is always lower than the total surface tension of separate phases due to interactions at the interphase. If one of the two phases in contact is non-polar, only dispersion interactions are possible. While dispersion forces exist in all molecules, the polar forces only exist in special molecules. They arise from the difference in electro negativity between different atoms in the same
Lenzinger Berichte, 82 (2003) 83-95 molecule [14]. Therefore one can determine the polar and disperse part of the surface free energy measuring the contact angle between a solid and several liquids of different polarity [14,27]. Material and methods Fibres and fibres treatment processes One solvent spun cellulose fibre (Lenzing Lyocell - CLY) and two conventional cellulose fibres made by the viscose process (Lenzing Viscose CV and Lenzing Modal CMD) were investigated and are summarized in Table 1. The fibres were treated using a Turbomat Ahiba laboratory dyeing apparatus according to the processes conventionally used in textile praxis (Table 2). The alkaline treatment was performed without any tension on the fibres (usually called slack mercerisation); we will refer to it as “mercerisation”. Fibre type Viscose Modal Lyocell Symbol CV CMD CLY Linear density 1.88 1.78 1.82 Tt [dtex] ±0.15 ±0.23 ±0.3 Fibre length 39.9 40.1 39.4 l [mm] ±0.51 ±0.33 ±0.44 Fibre diameter 14.3 14.2 12.8 d [µm] ±1.39 ±1.10 ±1.00 Density 1.5045 1.5141 1.5205 ρ [g/cm 3 ] Degree of polymerisation DPη Molecular mass Mη 235 ±5.13 38.000 ±880 507 ±3.61 82.100 ±540 642 ±4.58 104.000 ±730 Table 1. The specifications of investigated regenerated cellulose fibres X-Ray analysis The influence of different treatment processes, e.g. bleaching and slack mercerising (alkaline treatment), on the structural changes in different types of regenerated cellulose fibres was investigated by means of x-ray analysis, the procedure is described in detail in several references [24,1,6,2,29,34,35]. Ni filtered copper radiation from a conventional x-ray tube 85 (50 kV / 45 mA) was used in all scattering experiments. Bleaching Alkaline treatment 6 ml/l H2O2 40 g/l NaOH 2 ml/l Tanatex Geo 7 ml/l Tanawet BC (mineral stabilizer for (wetting agent, H2O2 stabilization) anionic) pH = 10.7 pH = 12.8 t = 30 min t = 1 min T = 98° C T = 10°C Table 2. Conditions of the treatment processes - bleaching and slack mercerisation (alkaline treatment without tension) Determination of the long spacing by small angle x-ray scattering (SAXS). SAXS intensity curves were measured using a Kratky camera with a slit collimation using a PSD position sensitive detector counting scattered intensity in the meridional direction, i.e. parallel to the fibre axis. The experimental data was corrected for absorption and background scattering and long spacing was determined applying Bragg’s law: n.λ L = (1) 2.sinθ where L is the long spacing, n order of reflection, λ wavelength of x-rays and θ Bragg’s scattering angle. Determination of the crystallinity index and crystalline orientation by wide-angle x-ray scattering (WAXS). A two-circle goniometer equipped with a linear position sensitive detector (PSD was used to measure the twodimensional WAXS scattering diagrams (azimuthal angular range from 0° to 180° in steps of 5°). The full pattern after the subtraction of the background scattering and the absorption and some steps of the evaluating procedure for one of the analysed fibres are presented in Figure 1. A weighted integration of the two dimensional diagram (Eq.2) was performed to obtain a „randomized“ scattering I(2θ) curve, which was corrected for Compton scattering.
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Correlation of regenerated fibres morphology and surface ... - Lenzing

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